Regulated Operations [Line Items]

No definition available.

Name:	nix_RegulatedOperationsLineItems
Namespace Prefix:	nix_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478742/980-340-50-1

+ Details

Name:	us-gaap_RegulatoryAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

+ Details

Name:	us-gaap_RegulatoryAssetsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

+ Details

Name:	us-gaap_RegulatoryAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Regulatory Matters (Regulatory Assets) (Details) - USD ($)	Dec. 31, 2025	Sep. 30, 2025	Dec. 31, 2024
Regulated Operations [Line Items]
Total Assets	$ 2,499,400,000	$ 5.7	$ 2,477,300,000
Regulatory Assets, Current	274,200,000		319,900,000
Regulatory Assets, Noncurrent	2,225,200,000		2,157,400,000
Unrecognized Pension Benefit And Other Postretirement Benefit Costs
Regulated Operations [Line Items]
Total Assets	433,100,000		485,300,000
Other Postretirement Costs
Regulated Operations [Line Items]
Total Assets	30,800,000		45,300,000
Environmental Costs
Regulated Operations [Line Items]
Total Assets	42,000,000.0		42,800,000
Regulatory Effects Of Accounting For Income Taxes
Regulated Operations [Line Items]
Total Assets	190,800,000		172,000,000.0
Underrecovered Gas And Fuel Costs
Regulated Operations [Line Items]
Total Assets	41,300,000		82,300,000
Depreciation
Regulated Operations [Line Items]
Total Assets	218,400,000		214,900,000
Post-In Service Carrying Charges
Regulated Operations [Line Items]
Total Assets	287,200,000		280,800,000
Safety Activity Costs
Regulated Operations [Line Items]
Total Assets	200,600,000		203,700,000
DSM Program
Regulated Operations [Line Items]
Total Assets	13,400,000		17,800,000
Retired coal generating stations
Regulated Operations [Line Items]
Total Assets	552,100,000		617,000,000.0
Deferred Derivative Gain (Loss)
Regulated Operations [Line Items]
Total Assets	10,100,000		6,500,000
Deferred Property Taxes
Regulated Operations [Line Items]
Total Assets	79,400,000		75,900,000
Renewable Energy Program
Regulated Operations [Line Items]
Total Assets	130,700,000		81,300,000
Other Assets
Regulated Operations [Line Items]
Total Assets	116,200,000		105,800,000
Work and Asset Management enterprise resourcing system
Regulated Operations [Line Items]
Total Assets	33,500,000		21,500,000
Customer Assistance Programs
Regulated Operations [Line Items]
Total Assets	17,300,000		24,400,000
Uncollected future cost of removal
Regulated Operations [Line Items]
Total Assets	$ 102,500,000		$ 0